The present invention relates generally to transformer inrush current reduction, and more specifically, to a system, apparatus, and method for reducing inrush current when energizing a three-phase transformer.
When energizing a three-phase transformer in an electric power delivery system (such as an electric power generation, transmission, or distribution system, or the like), inrush currents may occur which can be as large as ten times the transformer's nominal current and can last for up to around half of a second. The actual magnitude of these inrush currents depends on the impedance of the source supplying the transformer, the residual magnetic flux existing in the transformer, and the angle of the applied voltage at the time of energization.
Such high transformer inrush currents have a number of potentially adverse effects. For one, a high inrush current can significantly heat transformer windings and cause deterioration of insulation in the transformer. In addition, high inrush currents can place large mechanical stresses on the transformer windings sufficient to displace the windings on the transformer core, which, in the worst case, can break electrical connections within the transformer. Moreover, insulation compression from displacement of the transformer windings can result in turn-to-turn faults within the transformer, which, if left undetected, can eventually destroy the transformer.
Large inrush currents can also disrupt a power system by inappropriately tripping circuit breakers and over-current relays, by causing voltage sags which can affect sensitive equipment, by introducing large harmonic components, and by instigating sympathetic inrush currents in adjacent, parallel-connected transformers.
Some prior art inrush current reduction methods which have been previously used or suggested include 1) the use of resistors or other components temporarily switched in series or in parallel with the transformer windings; 2) the use of controlled voltage energization without accounting for the residual flux in the transformer; and 3) the use of controlled voltage energization based on an estimate of the residual flux in the transformer windings.
These prior methods have not been entirely satisfactory for use in power delivery systems because they have either required the temporary interposition of additional components in one of the circuits of the transformer during energization or have required circuit breakers capable of individual phase control with the possible need to measure residual flux levels in the transformer winding, which can be failure-prone or in some cases require significant upgrade of existing equipment.
The present disclosure provides a system, apparatus and method for effectively performing this pre-fluxing operation in a conventional three-phase transformer. In particular, the present disclosure is directed to a system, apparatus and method for establishing pre-flux levels in the three core segments associated with the transformer primary windings of a three-phase transformer such that when the windings are energized at the correct instant significantly reduced inrush current results. The system is automatic, requiring only user actuation prior to energization of the transformer.